Mikomoto vs Osezaki: Izu's Two Greatest Dive Sites Compared by Data
2026-03-11
Two Legends of Izu Diving
Mikomoto and Osezaki are the most iconic dive sites on the Izu Peninsula, both famous for big marine life encounters. But they couldn't be more different in character.
- Mikomoto: A small offshore island 4 km south of the Izu Peninsula tip, directly exposed to the Kuroshio Current. Famous for schooling scalloped hammerheads, manta rays, and massive schools of yellowback fusiliers. Accessed by boat only — requires calm seas.
- Osezaki: A cape site in Numazu City with four distinct dive zones (bay, open sea, tip, fence area). Welcomes everyone from absolute beginners (bay) to drift dive enthusiasts (open sea, tip).
We compared both using 5,600+ real dive observations.
Zone-by-Zone Data Breakdown
Osezaki is not a single site — its four zones have dramatically different visibility characteristics.
| Site | Type | Avg vis | Max | AI R² | Obs. |
|---|---|---|---|---|---|
| Mikomoto | Offshore island (open ocean) | 12.3m | 30m | 0.327 | 2,263 |
| Osezaki (open sea) | Open-sea point | 10.6m | 40m | 0.538 | 931 |
| Osezaki (tip) | Cape tip | 10.5m | 35m | 0.602 | 434 |
| Osezaki (bay) | Enclosed bay | 7.6m | 30m | 0.000 | 2,016 |
The enclosed bay is nearly completely isolated from wind and wave energy, making its visibility uncorrelated with external environmental variables (R²=0.000). Internal processes — phytoplankton growth cycles, micro-convection, and stratification breakdown — dominate, but these cannot be captured by weather data. It's truly a "you have to show up to find out" site.
Monthly Visibility: All Zones × 12 Months
| Month | Mikomoto | Osezaki (all) | Open sea | Bay | Δ (Miko−Ose) |
|---|---|---|---|---|---|
| Jan | 13.6 | 12.1 | 13.2 | 9.8 | +1.5m |
| Feb | 12.6 | 12.0 | 12.8 | 9.2 | +0.6m |
| Mar | 11.3 | 9.6 | 10.8 | 7.9 | +1.7m |
| Apr | 9.9 | 7.5 | 9.1 | 6.2 | +2.4m |
| May | 9.7 | 9.0 | 10.8 | 7.3 | +0.7m |
| Jun | 12.3 | 9.1 | 11.5 | 7.1 | +3.2m |
| Jul | 12.4 | 7.1 | 9.2 | 5.6 | +5.3m |
| Aug | 12.7 | 7.4 | 9.8 | 5.9 | +5.3m |
| Sep | 13.2 | 8.3 | 10.4 | 6.8 | +4.9m |
| Oct | 12.9 | 8.2 | 10.0 | 6.4 | +4.7m |
| Nov | 12.5 | 8.7 | 10.2 | 7.4 | +3.8m |
| Dec | 13.7 | 10.9 | 12.1 | 8.8 | +2.8m |
The Visibility Gap: Largest in Summer
Mikomoto maintains 12.4m in July while Osezaki (overall) drops to 7.1m — a gap of 5.3m. Osezaki Bay sinks to 5.6m in July, while Mikomoto stays 12.4m. That's nearly 7m difference between two sites less than 20 km apart.
The reason: Mikomoto's constant Kuroshio-driven water exchange prevents summer stratification from accumulating plankton. Osezaki Bay's enclosed nature traps nutrient-rich water; summer heat and sunlight trigger explosive phytoplankton blooms.
In winter, the gap narrows: Mikomoto 13.7m (December) vs Osezaki overall 12.1m (January) — only +1.6m difference. When the bay loses its thermal stratification in winter, its water clarity recovers significantly.
Summary
- Mikomoto (avg 12.3m) outperforms overall Osezaki (avg 8.8m) by +3.5m
- Largest gap in July: +5.3m (Mikomoto 12.4m vs Osezaki 7.1m)
- Osezaki open sea/tip zones (10.5–10.6m) are much better than bay (7.6m)
- Osezaki Bay AI R² = 0.000 — completely unpredictable; call the dive shop
- Osezaki Cape Tip has the best AI accuracy in the Izu Peninsula (R²=0.602)
- Best months: Mikomoto Dec (13.7m), Osezaki Jan–Feb (~12m)
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